1. Field of the Invention
The present invention relates to a rubber composition comprising silica and a silane coupling agent and to a pneumatic tire prepared by using the rubber composition, and more particularly, to a rubber composition in which cross-linking reaction (i.e., gelation) of a polymer due to a silane coupling agent during mixing at high temperature of 150.degree. C. or more is suppressed such that the reaction of silica and the silane coupling agent proceeds efficiently without a deterioration in workability.
2. Description of the Related Art
Heretofore, carbon black has been used as a reinforcing filler for rubber because carbon black provides higher reinforcement and more excellent abrasion resistance than other fillers. Recently, because of social requirements to save energy and to save resources, particularly to cut down fuel consumption of automobiles, a decrease in the heat buildup of rubber compositions is also required.
For decreasing the heat buildup of rubber compositions by using carbon black, use of a small amount of carbon black or carbon black having a large particle size is considered. It is, however, well known that, in both methods, decreasing heat buildup is in a contradictory relation with improving reinforcement and abrasion resistance of a rubber composition.
On the other hand, silica is known as a filler which provides decreased heat buildup of a rubber composition, and applications for many patents, for example, Japanese Patent Application Laid-Open No. Hei-3-252431, have heretofore been made.
However, silica particles tend to cohere together due to hydrogen bonding of silanol groups which are functional groups on the surfaces of the silica particles. For improving the dispersion of silica particles into rubber, the mixing time must be increased. When dispersion of silica particles into rubber is insufficient, a problem arises in that processability in processes such as extrusion and the like deteriorates due to the increase in the Mooney viscosity.
Moreover, the surfaces of the silica particles are acidic. Therefore, there are problems in that basic substances used as vulcanization accelerators are absorbed such that vulcanization is not carried out sufficiently, and a sufficient modulus of elasticity is not obtained.
In order to solve these problems, various types of silane coupling agents have been developed. For example, use of a silica-silane coupling agent as a reinforcing material is disclosed, for example, in Japanese Patent Application Publication No. Sho-51-20208. As the silane coupling agent, Si69 (trade name), which is manufactured by DEGUSSA AG., a German company and has a structure expressed by the following formula, is widely used: EQU (C.sub.2 H.sub.5 O).sub.3 Si(CH.sub.2).sub.3 Sa(CH.sub.2).sub.3 Si(OC.sub.2 H.sub.5).sub.3
wherein a represents a positive number of 1 to 9. Here, the silane coupling agent is a mixture in which a is a number of 1 to 9, in which the average number of sulfur atoms is normally about 4, and which contains a relatively large amount of high polysulfide silane in which a is a number of 5 or more.
However, when the mixing temperature of rubber is low, the sufficient reinforcing effect is not obtained by this type of silane coupling agent. Moreover, the silane coupling agent is hydrolyzed, and the generated ethanol does not vaporize sufficiently and vaporizes during extrusion. Thus, there is a drawback in that blisters are formed. On the other hand, when mixing is conducted at high temperatures of 150.degree. C. or more, the reinforcing property is improved. However, as a drawback, gelation of the polymer caused by the silane coupling agent takes place during mixing, and the Mooney viscosity increases. Thus, there is a drawback in that processing in later stages becomes difficult.
Therefore, as a result of studies conducted by the present inventors on a silane coupling agent which is suitable for mixing at a high temperature, it was found that, in order to prevent gelation of the polymer, it is necessary to reduce the content of high polysulfide silane such as pentasulfide silane, heptasulfide silane, hexasulfide silane, and the like, in the polysulfide silane.
However, when the content of sulfur in the polysulfide silane is changed, simply in accordance with the following reaction formula (A): ##STR1##
(wherein, Et represents an ethyl group), the entire distribution of number of sulfur is shifted and the content of disulfide silane which has a poor coupling effect is increased. Accordingly, a method of selectively removing high polysulfide silane is required.